This invention relates to an automatic compensator for the working clearance of the shoes of a drum brake for a motor vehicle.
The European patent EP-B-O 538 909 describes an automatic compensator of the type defined in the preamble to Claim 1.
It is known that expansion of the brake drum can cause the mechanism to overcompensate when the brake is hot; if this is permitted to occur, all working clearance between the brake shoe liners and the drum could be lost, meaning that, when the brake cooled down, the liners would cause resistance, braking the wheels. Clearance must therefore be adjusted at relatively low temperatures and compensation must be prevented at high temperatures.
European patent EP-B-O 538 909 suggests providing the compensator with heat-sensitive means operable to prohibit compensation caused by thermal expansion. In the arrangement proposed in this patent, the heat-sensitive means include a bi-metal strip with one end fixed to a tubular insert slidable and rotatable on an intermediate sleeve member into which two rod elements to be connected to the brake shoes are inserted coaxially. The other end of the bi-metal strip has a precision-shaped head engageable in a locking seat formed in an insert securely fixed to one of the two rod elements when the temperature rises above a predetermined threshold. In these conditions, the compensator behaves like a rigid unit which is prevented from lengthening in order to stop a ratchet mechanism from shifting and adjusting the minimum resting length of the compensator.
This arrangement involves a drawback in that it requires the manufacture and fitting of small parts which must be of a very precise shape, and dimension in order to operate correctly at high temperatures. In particular, the head of the bi-metal strip and the seat thereof, formed in an insert fitted into one of the aforesaid rod elements, demand very high manufacturing and assembly tolerances, which can only be achieved in complex and costly operations.
The object of the present invention is to provide an improved automatic compensator while containing manufacturing costs.
This and other objects and advantages, which will be better understood later, are achieved, according to the present invention, by providing an automatic compensator having the characteristics claimed in the characterising portion of Claim 1.
Other important characteristics are described in the dependent Claims.
A clearance compensator for the shoes of a drum brake for motor vehicle, of a type including: three coaxial elements, comprising a first rod element with a head portion for fixing against rotation to a first brake show and a threaded stem portion, an intermediate element threadably engaged to the said first rod element, a second rod element with a head portion for fixing non-rotatably to a second brake show and a stem portion slidably connected to the intermediate portion so as to be freely rotatable with respect thereto; a tubular insert rotatably mounted to the intermediate element for sliding axially thereon and capable of reaching an axial stop position on said intermediate element; a ratchet wheel secured for rotation with the intermediate element between the second rod element and the tubular insert; a leaf spring acting between the second rod element and the tubular insert so as to urge the second rod element away from the intermediate element, the said leaf spring carrying a stop element for engaging the ratchet wheel; heat-sensitive be-metal strip means for preventing the ratchet wheel from rotating when the temperature of the brake exceeds a predetermined value; characterized in that the said bi-metal strip means include a bi-metal strip fixed so as to project from the leaf spring, orientated so as to be substantially tangential to the ratchet wheel and having the said stop element at the free end thereof; the said strip having a normal operating position in which the stop element engages the ratchet wheel and a heat-inflected position in which the stop element is disengaged from the said ratchet wheel at temperatures exceeding a predetermined value.